Clams

SPECIES
The species that now dominates clam farming in BC is the Manila clam (Tapes philippinarum). Like the Pacific Oyster, Manila clams are not indigenous to the Pacific Northwest but were accidentally introduced.  Some clam seed was inadvertently included with Pacific Oyster seed brought in from Japan in the 1930’s. The first specimens were found in Ladysmith Harbour in 1936.  It lives along much of the BC coast as far north as Bella Bella.  It occupies an area higher in the intertidal zone and does not burrow as deeply as native species such as Butter clams (Saxidomus giganteus) and Littleneck clams (Protothaca staminea).  It has managed to co-exist well with its other clam species while supporting both a wild clam harvest and a growing aquaculture industry.

Manila clams have been harvested along the British Columbia coast since the 1970’s but only since 1985 have they been farmed. Techniques developed in Washington for clam culture were adapted for British Columbia and the industry has continued to expand since then.

SEED PRODUCTION
The production cycle for Manila clams begins with the hatchery production of larvae. Hatheries maintain broodstock for the production of clam seed sold to growers. Most hatcheries will aim to have seed available in the spring but it is up to the grower to make sure orders are placed well in advance. To plan production, hatcheries must begin conditioning broodstock for spawning in winter. Spawning is induced by manipulating temperature and feed. The larvae are grown for two weeks until they set and may be reared for a further period in the hatchery until they are large enough to be shipped to growers. Experimental methods to set clam larvae on specially prepared intertidal sites are under development. For details on clam seed go the seed page.

NURSERY REARING
Growers who choose to buy post-set juvenile clams in the spring will not usually plant them directly on the beach.  They will be boosted to a larger size under more controlled and gentle conditions.  Hatcheries will often undertake the first stage of nursery rearing in indoor upwellers, which can be used to raise seed from the time they have set (somewhere around 0.2mm in size) up to the specific sizes growers can purchase (e.g. 2-3mm, 3-4mm, 4-6mm, 6-9mm).  Growers may build their own upwellers, either on land or floating (FLUPSY), to boost the seed further after purchase.  The size at which to purchase seed depends on the rearing system to be used.  However, it should be remembered that the lower cost of smaller seed can easily be offset by higher mortalities.  Many growers prefer to start with larger seed which have higher survival rates thus making production more predictable.  One method is to use a combination of spat bags held in tray stacks or trays lined with fiberglass window screen.  How large to grow the seed in the nursery depends on when and how the grower plans to put them out on the intertidal beach growout area.  They can be reared in off-bottom systems up to a size of 20mm before having to be planted. If they are grown larger there will be shell erosion and mortality.  For details see nursery page.

GROWOUT
Prior to seeding the beach with clams, the area must be properly prepared (assuming the site meets suitable biophysical criteria).  Just as fields for agriculture are plowed, disked and harrowed, plots for clam culture require substrate enhancement, measures to minimize storm and wave damage, and predator control.  The clams are removed from the nursery system and spread at a specific density on the prepared subtidal plots.  If the seed is started in the spring this may be done in the fall.  In other cases seed may be nursery reared through the winter and outplanted on the beach the following spring or summer.  Once seeded on the beach, the clams will burrow into the substrate, ideally a mix of sand and pea gravel.

It is recommended that the area be covered with a plastic mesh material to protect the clams from ducks, crabs and other predators.  The netting can be secured by digging the perimeter into the substrate, staking it down with rebar or cedar stakes or putting large rocks around the mesh perimeter.  If it is staked or weighted with rocks a lead line can be threaded around the perimeter to keep the edges from floating up and allowing predator access.  In many instances, clam farms will benefit from some natural recruitment of Manila clams, enhancing production and offsetting some of the inevitable stock mortality.  For details see the intertidal page.

HARVEST
Depending on site characteristics, clams may take from 2 to 4 years to reach marketable size.  To harvest clams, the predator cover (if present) is removed and clams are dug out of the substrate and collected.  This is still usually done by hand using a long-tined rake.  The substrate is turned with the rake and clams are brought to the surface in the process.  They are hand-picked and collected into mesh bags.  A tined scoop has been devised by one grower to dig and grade clams from areas with poor tidal drainage (see the harvest page).  Mechanical harvesters are under development in BC and a couple of prototypes have been tested.

The harvested clams are generally wet stored, either by the processor or on the grower’s site, for at least 24 hours to purge sand.  They can be wet stored longer, in the low intertidal or in sink floats, until ready for delivery.  For beach storage mesh bags of clams should be set out in a single layer on firm substrate and turned and gently shaken daily.

Yields of clams on well-managed farm plots can reach to over 2kg per square meter each year. Wild harvested clams on average yield only 0.05 to 0.1kg per square meter per year.

While Manila clams will likely continue to be the mainstay of clam farming in BC, culture of another clam species is currently under investigation: the large and valuable Geoduck clam (Panope generosa). Development of subtidal grow-out methods may soon lead to commercial viability.

PRINTED SOURCES:

  • Manila Clam Culture – Pilot Scale Site and Husbandry Assessment, 1990 (BCMAFF Bulletin 34 – AFF0385).
  • Manila Clam Nursery and Growout Operations in Washington State, 1990 (BCMAFF Bulletin 32 – AFF0338).
  • Proceedings of the 1990 Manila Clam Culture Workshop, 1990 (BCMAFF Aquaculture Industry Development Report 90-09, AFF0407).
  • Manila Clam Aquaculture in British Columbia: Information for Getting Started, 1992 (BCMAFF Aquaculture Industry Development Report 92-01, AFF2899).
  • Estimated Costs and Returns for a Clam Grow-out Enterprise, 1993 (BCMAFF Aquaculture Industry Development Report 93-05, AFF3107).
  • Estimated Costs and Returns for a Seeded Clam Grow-out Enterprise, 1993 (BCMAFF Bulletin 40 – AFF1749).
  • Shellfish Culture Capability Appraisals for Specific Regions: Okeover Inlet (1995 BCMAFF Publication #AFF3261), Nootka Sound (1995 BCMAFF Publication #AFF3260), Kyuquot Sound (1995 BCMAFF Publication #AFF3259), Quatsino Sound (1995 BCMAFF Publication #AFF3258, Clayoquot Sound (1993 BCMAFF Publication #AFF2847).
  • Cross, S.F. and Kingzett, B.C. 1992. Biophysical Criteria for Shellfish Culture in British Columbia: A Site Capability Evaluation System. (BCMAFF Publication #AFF2965)
  • Kingzett, B.C 1995. Towards Quality Assurance: An Information Manual for Shellfish Growers in British Columbia. (BCMAFF Publication AFF3244)
  • Brian Kingzett and Neil Bourne 1995. The clam industry of British Columbia. The Westcoast Fisherman, September.

WEBSITES:

  • Fisheries and Oceans Canada:Species and Habitat of Shellfish.
  • Fisheries and Oceans Canada:Diseases and Parasites of Shellfish